One of the major problems encountered in the waterflooding of permeability-stratified reservoirs is the preferential flow of water through the more permeable zones between injector and producer wells. This preferential flow greatly reduces the sweep efficiency of driving fluids. This reduction in sweep efficiency can also occur in steam and miscible CO.sub.2 -flooding processes.
To improve sweep efficiency, the permeability of such zones must be reduced. This technique is commonly known as permeability profile control. Methods for plugging off, diverting, or reducing the rate of undesired fluid movement in porous media make up a substantial amount of the technology, including placing gels in the formation. Such gels are used to plug highly permeable zones in the formation, thus diverting the water or other fluid through the less permeable zones, thereby improving sweep efficiency and providing greater oil recovery. These prior art gels degrade when sheared, as during the pumping operation through pipes, perforations, and the permeable zones in the formation, resulting in the breakdown of gel structures and the loss of gel's ability to plug and maintain impermeability. Therefore, they cannot be prepared on the surface and then pumped underground into the formations. Instead, the gellation must be done "in situ" within the formation. Polysaccharide biopolymers, such as xanthan gum, cellulose derivatives, guar gum, etc., are useful for reservoir permeability profile control in the crosslinked gel-forms. Chromium crosslinked xanthan gum has been successfully used in many fields to recover incremental oil. Cr-xanthan gel has many unique features such as brine tolerance, shear stability, shear thinning, and rehealing of the sheared gel. An important advantage of the Cr-xanthan gel which derives from these shear properties is that it can be prepared on the surface and then pumped underground into the formations. A major deficiency of Cr-xanthan and other biopolymers is their low thermal stability. Xanthan gum application is limited to wells with temperatures under 150.degree. F. However, there are many reservoirs with higher temperatures. Thermal stability of xanthan gum must be improved in order for these materials to be used to treat reservoirs having high temperatures.
It has been found that amino-resins can react with xanthan gum to result in either gelled or solution form to produce a more thermally stable material. Further reaction with chromium or other metals produces thermally stable, brine tolerant, shear thinning, rehealable gels suitable for high temperature reservoir uses.